Liquid metal catalyst for biodiesel production

ABSTRACT

Molten metal or molten metal alloy catalysts for transesterification reactions are provided. In particular, readily available molten tin is used as a catalyst for biodiesel production. Catalysts comprising tin alloys with low melting point temperatures to allow for low operation temperatures are also provided. The molten catalysts remain in the reaction zone and do not contaminate the product stream, thereby product separation and recovery are manageable. By using molten tin catalysts, it is possible to produce a high quality glycerol as a second valuable product. Furthermore, wide quality ranges of reactants (alcohol and oil) are available for biodiesel production with molten tin catalysts.

FIELD OF THE INVENTION

The invention relates generally to biodiesel production. More particularly, the present invention relates to catalysts for biodiesel production.

BACKGROUND

In recent years, biodiesel production has increased nationwide. Biodiesel production generally includes the reaction of alcohol and oil to produce esters (biodiesel) and glycerol. The amount of biodiesel currently produced, however, can only accommodate a small fraction of the total amount of diesel consumed. Methods and processes to more effectively produce biodiesel from a wider range of feedstocks would enable greater amounts of biodiesel production, thereby making biodiesel use more widespread.

The major problem in the biodiesel production market is the cost of feedstock, with other substantial costs due to the polishing of the product and the treatment of waste process streams. In particular, well-established methods of producing biodiesel rely on the use of a homogeneous consumable catalyst, such as aqueous sodium or potassium hydroxide, in a batch-wise operation. These traditional methods, however, are restricted by the use of oils with low free fatty acid concentrations, generally only found in expensive feed stocks, such as palm, canola, or soybean oil. In addition, traditional biodiesel production methods require expensive and time-consuming process steps to remove excess catalyst from the biodiesel and glycerol products. Excess alcohol is another product contaminant and must be removed. Though glycerol can be a valuable secondary product, the glycerol produced using traditional catalysts is often of low quality and has little market value. Furthermore, the batch-wise operation requires long reaction times in multiple large reactors, which create significant variation in product quality and composition. The large product quality variations dictate continuous and costly product analysis.

Several non-catalytic and heterogeneous catalytic reaction systems have been developed to improve on the traditional methods of transesterification, such as by allowing continuous instead of batch-wise operation. Flexible heterogeneous catalysts are generally in a powder or pellet form, or are coated on the reactor walls. Though these methods reduce some of the difficulties associated with traditional biodiesel production methods that rely on consumable liquid catalysts, they generally require high operational temperatures and pressures. Certain heterogeneous techniques even require supercritical conditions during the reaction.

Extreme reactor conditions create many difficulties for heterogeneous catalytic and non-catalytic reaction systems, including the necessity of large alcohol-to-oil ratios, specialized materials, and unwanted by-products side reactions. Large alcohol-to-oil ratios and by-products production demand high recovery and removal costs, respectively. In addition, the elevated temperatures and pressures create large energy demands, which increases the energy costs and decreases the environmental value of biodiesel production over standard diesel production.

The present invention addresses the problems of current biodiesel production and advances the art with transesterification and esterification methods that utilize a novel catalyst.

SUMMARY OF THE INVENTION

The present invention is directed to molten metal or molten metal alloy catalysts for biodiesel production. The method of producing biodiesel through transesterification according to the present invention includes introducing multiple reactants into a reaction zone having a catalyst, where the reactants include at least one feedstock oil and an alcohol. The catalyst includes a molten metal or a molten metal alloy for catalyzing the reaction between the reactants to form biodiesel and glycerol.

In a preferred embodiment, the catalyst includes molten tin. In another preferred embodiment, the catalyst includes molten tin alloyed with at least a second metal, such as lead, indium, bismuth, copper, antimony, silver, or any combination thereof. The alloy preferably has a lower melting point than the melting point of unalloyed tin. In an embodiment, the feedstock oil is selected from the group consisting of tri-glycerides, di-glycerides, mono-glycerides, and free fatty acids. In an embodiment, the alcohol is methanol, ethanol, propanol, butanol, another short chain (e.g. C1 through C6) linear alcohol, or a branched alcohol. In yet another embodiment, the reactant alcohol includes “wet” ethanol.

The method of the present invention also includes removing the products from the reaction zone, wherein all of the catalyst remains in the reaction zone. In a preferred embodiment, the reaction zone is heated to a reaction temperature that is greater than or equal to the melting point of the catalyst. In an embodiment, the operating pressure of the reaction zone is greater than or equal to one atmosphere of pressure. In a preferred embodiment, the molar ratio of the alcohol to the feedstock oil ranges from 3:1 to 50:1.

The present invention is also directed to methods of catalyzing a transesterification reaction, whereby a first alcohol and a first ester (e.g. mono, di, or tri-glyceride) reacts to form a second alcohol and a second ester (or group of esters dependant on the fatty acid composition of the first ester). A molten metal or molten metal alloy catalyst, such as tin or tin alloy, catalyzes the transesterification reaction. In addition to transesterification, the molten metal or molten metal catalyst can be used to catalyze reactions to produce an ester from a carboxylic acid, such as a free fatty acid.

BRIEF DESCRIPTION OF THE FIGURES

The present invention together with its objectives and advantages will be understood by reading the following description in conjunction with the drawings, in which:

FIG. 1 shows an example process flow diagram for biodiesel production based upon the catalyst according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In recent years, interest in alternative and renewable energy sources has grown. Biodiesel is a promising carbon neutral alternative to traditional diesel. However, producing economically competitive biodiesel remains a daunting task. The present invention is directed to biodiesel production using a molten metal or metal alloy catalyst for improved production.

Biodiesel production from triglycerides generally involves a transesterification reaction, in which three molecules of alcohol and a single molecule of oil react to form one molecule of glycerol and 3 molecules of esters, i.e. a first ester (triglyceride) is converted to multiple second esters (mono-alkyl esters) which collectively are called biodiesel. In commercial production, methanol is most often used as the reactant alcohol and expensive partially refined oils, such as palm, canola, or soybean, are used as the reactant oil. The product biodiesel is typically a mixture of mono-alkyl esters. A secondary product, glycerol, is also generally produced by the reaction. It is noted that an ester can also be produced by reacting alcohol and carboxylic acid instead of feedstock oil.

Reactions for biodiesel production are often catalytic and various types of catalysts have been developed to improve reaction efficiency, time, and costs. The present invention is directed to the use of a molten metal or a molten metal alloy as a catalyst, preferably for biodiesel production. In a preferred embodiment, the catalyst is molten tin. Tin has the desirable traits of being readily available and inexpensive, particularly relative to other traditionally used catalysts. Additionally, the tin catalyst is not depleted or deactivated during the course of reaction.

In another preferred embodiment, the catalyst is molten tin alloyed with at least a second metal. Though any tin alloy can be used in the present invention, alloys having a lower melting point than pure tin are particularly advantageous by lowering operational and energy costs. Pure or unalloyed tin has a melting point of approximately 232° C. at atmospheric pressure. In certain embodiments, the second metal is selected from the group consisting of lead, indium, bismuth, copper, antimony, silver, or any combination thereof. It is noted that any number of elements can be used in the alloy. The addition of secondary, tertiary, etc. metals can also be used to change the selectivity and activity of the catalyst. An example alloy comprises about 48% Tin and about 52% Indium and has a melting point of 118° C. Another example alloy comprises 92.5% Tin, 3.5% Silver, 1% Copper, and 3% Bismuth and has a melting point of 213° C.

In contrast to traditional catalysts, the use of molten metal or molten metal alloy catalysts enable a wider variety of feedstock oils to be used as reactants. By catalyzing with molten metal or metal alloy, the reaction is not limited by fatty acid or moisture content in the feedstock oil. In particular, feedstock oils of varying quality, from expensive refined oils to waste oils, can be used as reactants. In an embodiment, the reactant feedstock oil is selected from the group consisting of tri-glycerides, di-glycerides, mono-glycerides, and free fatty acids. In particular, the free fatty acid reactants can be derived from at least one, but not limited to, the following: Animal sources including raw or rendered animal fats, yellow grease, white grease, brown grease, tallow or lard from pork, beef, chicken, mutton or combinations thereof, or plant sources including legumes, corn, olives, safflower, palm fruit, palm nut, mustard seed, canola, coconut, tall oils, cotton, linseed, castor bean, or algae.

In an embodiment of the present invention, the reactant alcohol is a short chain linear alcohol or a branched alcohol. In particular, the reactant alcohol is selected from the group consisting of methanol, ethanol, propanol, and butanol. Molten metal or molten metal alloy catalysts also allow the use of less expensive and/or less pure alcohol. In an exemplary embodiment, “wet” ethanol is used as a reactant. Wet ethanol is a mixture of ethanol and water with less than or equal to 95.6% ethanol.

FIG. 1 shows an example process flow diagram for biodiesel production according to the present invention. The alcohol feed storage 101 and the oil feed storage 102 are connected to a reactor 103 for introducing the reactant oil and alcohol to the reaction zone. The reactants are mixed by a mixer 104 and react with the help of the catalyst to form biodiesel and glycerol. The reactor 103 contains the catalyst. Known liquid-vapor or liquid-liquid contacting equipment can be used in the reactor. The products are then separated in a first separation stage 105 and a second separation stage 106. Finally, the separated products are separately placed in an ester (biodiesel) storage 107 and glycerol storage 108. Though FIG. 1 shows only one process flow for an example biodiesel production facility, it is noted that the molten catalyst of the present invention is applicable for any biodiesel production or transesterification facility.

It is important to note that the catalyst of the present invention remains in a molten state, therefore must be heated above its melting point. In particular, the reaction zone is heated above the melting point of the catalyst. In an embodiment, the reaction temperature has an upper limit of about 350° C., above which the reactants and desired reaction products begin to break down. FIG. 1 shows heating elements to achieve and/or maintain the catalyst in a molten state. Heating elements include startup heaters 109, makeup heater modules 110, and heat recover modules 113. Other elements shown by FIG. 1 include cooling water 111 and a shutdown oil bypass 112.

In addition to the reaction temperature, the operating pressure in the reaction zone can be established for improved reaction efficiency. In an embodiment, the operating pressure is greater than or equal to atmospheric pressure. One advantage of the molten metal process over traditional supercritical biodiesel production methods is that its thermodynamic reaction conditions are less severe. This allows for cost savings in thinner-walled tanks, reactors, piping, and other equipment, as well as savings in operating costs by requiring less energy input.

The present invention has numerous other advantages over existing biodiesel production processes. The molten catalyst does not leave the reaction zone as the products exit from the reactor. This behavior is similar to heterogeneous catalysts and is likely due to the large density difference and immiscibility between the molten catalyst and the reactants and products in the reaction zone. Tests have been conducted to verify that the catalyst remains in the reactor and no amounts of catalyst contaminate the product stream (to the parts per billion level).

Since the catalyst is not consumed during the reaction and does not leave the reactor, it is ideally suited for a continuous process. Demonstrations have indicated that continuous biodiesel production processes are achievable with a 95% conversion rate and a production time of about 3-10 minutes. Furthermore, the separation of the products is simplified by the lack of the catalyst in the product stream. Product separation is particularly important since glycerol is a secondary product that can be valuable. Though high quality glycerol can have great value, lower grade glycerol would be a waste byproduct. Using molten catalysts of the present invention enables a process that produces high quality glycerol. The quality of the glycerol also depends on the quality of the reactant feedstock oil. Furthermore, the molten metal or metal alloy catalyzed process does not produce soap or sodium methoxide as product contaminates, which typically result from traditional base catalyzed methods.

Another advantage of the present invention is the large range of alcohol to oil molar ratios that can be applied. In an embodiment of the present invention, the molar ratio of the alcohol to the feedstock oil ranges from 3:1 to 50:1 (or in the case of free fatty acid esterification 1:1 to 16:1). In another embodiment, the molar ratio is greater than 50:1. Although larger alcohol ratios typically provide extra product yield, the benefit of the additional yield is mitigated by large downstream separation and recovery costs for the alcohol. Since the molten catalyst of the present invention does not leave the reaction zone, separation of products is manageable and high alcohol to oil molar ratios can be utilized. In addition, the recovery cost of the alcohol is low since the temperature that the system operates allows for simple recovery processes, such as flash distillation.

As one of ordinary skill in the art will appreciate, various changes, substitutions, and alterations could be made or otherwise implemented without departing from the principles of the present invention, e.g. molten metal or metal alloy catalyst can be used for reactions involving reactants not listed above and usable for biodiesel production. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents. 

1. A method of producing biodiesel through transesterification, said method comprising: (a) introducing multiple reactants into a reaction zone, wherein said reactants comprise at least a feedstock oil and an alcohol; (b) having a catalyst in said reaction zone, wherein said catalyst comprises a molten metal or a molten metal alloy; and (c) reacting said multiple reactants to form biodiesel and glycerol in said reaction zone, wherein said reaction is catalyzed by said catalyst.
 2. The method as set forth in claim 1, wherein said catalyst comprises molten tin.
 3. The method as set forth in claim 1, wherein said catalyst comprises molten tin alloyed with at least a second metal.
 4. The method as set forth in claim 3, wherein said second metal is selected from the group consisting of lead, indium, bismuth, copper, antimony, and silver.
 5. The method as set forth in claim 3, wherein said molten tin alloyed with said second metal has a lower melting point than the melting point of unalloyed tin.
 6. The method as set forth in claim 1, further comprising removing said products from said reaction zone, wherein all of said catalyst remains in said reaction zone.
 7. The method as set forth in claim 1, wherein said feedstock oil of said reactants is selected from the group consisting of tri-glycerides, di-glycerides, mono-glycerides, and free fatty acids.
 8. The method as set forth in claim 1, wherein said alcohol of said reactants comprises a short chain linear alcohol or a branched alcohol.
 9. The method as set forth in claim 8, wherein said alcohol is selected from the group consisting of methanol, ethanol, propanol, and butanol.
 10. The method as set forth in claim 1, wherein said alcohol of said reactants comprises wet ethanol.
 11. The method as set forth in claim 1, further comprising heating said reaction zone to a reaction temperature, wherein said reaction temperature is greater than or equal to the melting point of said catalyst.
 12. The method as set forth in claim 1, further comprising establishing an operating pressure in said reaction zone, wherein said operating pressure is greater than or equal to atmospheric pressure.
 13. The method as set forth in claim 1, wherein the molar ratio of said alcohol to said feedstock oil ranges from 3:1 to 50:1.
 14. A method of catalyzing a transesterificaton reaction, said method comprising: (a) introducing multiple reactants into a reaction zone, wherein said reactants comprise a first alcohol and a first ester; (b) having a catalyst in said reaction zone, wherein said catalyst comprises a molten metal or a molten metal alloy; and (c) reacting said multiple reactants to form multiple products, wherein said products comprise a second alcohol and a second ester, wherein said second ester is different from said first ester, and wherein said reaction is catalyzed by said catalyst.
 15. A method of producing an ester from a carboxylic acid, said method comprising: (a) introducing multiple reactants into a reaction zone, wherein said reactants comprise said carboxylic acid and an alcohol; (b) having a catalyst in said reaction zone, wherein said catalyst comprises a molten metal or a molten metal alloy; and (c) reacting said multiple reactants to form a product, wherein said products comprise said ester and water, and wherein said reaction is catalyzed by said catalyst. 